Conventionally, various types of radar devices for transmitting a pulse signal to a detection area and detecting a target object based on a reflection signal have been proposed. Such radar devices mostly require large electric power by utilizing a magnetron, and use a signal having a particular pulse width according to a set display range. However, for semiconductor radars which utilize a semiconductor as an oscillation device, because their transmission powers are very low as compared with the magnetron radars, they must have a longer pulse width of the pulse signal to transmit. In addition, if performing a long-distance detection, the pulse width must typically be longer than the case where a short-distance detection is performed. For these reasons, if the semiconductor radar performs a long-distance detection, it must transmit a pulse signal having even longer pulse width. Using the pulse signal having a longer pulse width in turn reduces a distance resolution and increase a blind area which cannot be detected.
In order to improve such reduction in the distance resolution, a technique in which modulation is applied to a transmission signal to pulse-compress the signal is generally used. Meanwhile, in order to suppress the blind area, JP 2008-527391 discloses a method of transmitting a long pulse signal for long-distance detection (a long pulse) together with two or more kinds of pulse signals whose pulse widths are shorter than the long pulse signal (an middle pulse signal and a short pulse signal).
Further, as for the resolution to the low transmission power of semiconductor radar, if a single pulse signal, which has a long pulse width as described above, is transmitted, an S/N ratio deteriorates, and thus, JP 3639124 (B) discloses a pulse integral method in which two or more pulse signals of the same kind are transmitted continuously, and two or more reflection signals (received signal) based on the pulse signals are integrated to be used for detection.
However, in order to continuously transmits signals having the same pulse width and then perform pulse integration of received signals for the transmission, if a radar device using the method of JP 3639124 (B) is applied to the method of JP 2008-527391 in which the short pulse signal, the middle pulse signal, and the long pulse signal are sequentially transmitted, the following situation arises.
FIG. 5 is a timing chart showing the case where transmission of the short pulse signal, the middle pulse signal, and the long pulse signal is repeated for three times. FIG. 6 is a schematic diagram showing the situation occurred when the transmission of FIG. 5 is performed.
In this case, the radar device first sequentially transmits short pulse signals PS11, PS12, and PS13 which constitute a short pulse group PSG1, and then sequentially transmits middle pulse signals PM11, PM12, and PM13 which constitute an middle pulse group PMG1. The radar device further sequentially transmits long pulse signals PL11, PL12, and PL13 which constitute a long pulse group PLG1. The radar device repeats the sequential transmission of the short pulse group PSG, the middle pulse group PMG, and the long pulse group PLG.
Then, the radar device performs pulse integration of received signals for each pulse group. For example, the radar device performs short-distance detection by performing pulse integration using the received signals caused by the short pulse signals PS11, PS12, and PS13. Similarly, the middle-distance detection is performed by carrying out pulse integration using the received signals caused by the middle pulse signals PM11, PM12, and PM13, and the long-distance detection is performed by carrying out pulse integration using the received signals caused by the long pulse signals PL11, PL12, and PL13.
As described above, by the time the device obtains one set of the pulse integration results, the device must transmit and receive the signals for three times for each pulse group, and therefore, a relatively long period of time is required to obtain the pulse integration results for all the detection distance ranges.
Further, in such a radar device, in order to detect all azimuth directions, its antenna rotates continuously at a predetermined speed. For this reason, if the pulse signals are sequentially transmitted and received for each detection distance group as shown in FIG. 5, a rotating angle of the antenna during the transmission and reception for one detection distance group will be greater than the case where a single pulse is used. Thereby, as shown in FIG. 6, an azimuth range ΔθM1+ΔθL1 in which the middle-distance detection and the long-distance detection are performed intervenes between azimuth ranges ΔθS1 and ΔθS2 in which two short-distance detections are performed. This causes the short-distance detection to be inhibited during the period during which the middle-distance detection and the long-distance detection are performed. The same can be said for each of the middle-distance detection and the long-distance detection. That is, an azimuth range where large-range detection cannot be performed will occur for each detection distance area.